EP1637353A1

EP1637353A1 - Pneumatic tire having a tread containing immiscible rubber blend and silica

Info

Publication number: EP1637353A1
Application number: EP05108389A
Authority: EP
Inventors: Marc Weydert; Christian Kaes
Original assignee: Goodyear Tire and Rubber Co
Current assignee: Goodyear Tire and Rubber Co
Priority date: 2004-09-17
Filing date: 2005-09-13
Publication date: 2006-03-22
Anticipated expiration: 2025-09-13
Also published as: EP1637353B1; CN1765979A; DE602005027784D1; US7441572B2; BRPI0503643A; CN100415818C; JP5255176B2; US20060060285A1; JP2006083393A

Abstract

The invention is directed to a pneumatic tire having a tread comprising a vulcanizable rubber composition comprising, expressed as parts by weight per 100 parts by weight of elastomer (phr),

(A) 60 to 95 phr of solution-polymerized styrene-butadiene rubber with a bound styrene content of from 35 to 45 percent by weight, a vinyl 1,2 content of from 15 to 30 percent by weight based on the butadiene content, and a Tg of from -40°C to -20°C;
(B) 5 to 40 phr of cis-1,4 polybutadiene having a Tg of from -95°C to - 105°C;
(C) 50 to 150 phr of silica;
(D) 30 to 70 phr of processing oil; and
(E) 0.5 to 20 phr of a sulfur containing organosilicon compound.

Description

Background of the Invention

It is highly desirable for tires to have good wet skid resistance, low rolling resistance, and good wear characteristics. It has traditionally been very difficult to improve a tire's wear characteristics without sacrificing its wet skid resistance and traction characteristics. These properties depend, to a great extent, on the dynamic viscoelastic properties of the rubbers utilized in making the tire.
In order to reduce the rolling resistance and to improve the treadwear characteristics of tires, rubbers having a high rebound have traditionally been utilized in making tire tread rubber compounds. On the other hand, in order to increase the wet skid resistance of a tire, rubbers which undergo a large energy loss have generally been utilized in the tire's tread. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubbery material for automobile tire treads.

Summary of the Invention

The present invention is directed to a pneumatic tire according to claim 1. Dependent claims cover preferred embodiments of the invention.

Description of the Invention

The invention is directed to a pneumatic tire having a tread comprising a vulcanizable rubber composition according to claim 1.
Suitable solution polymerized styrene-butadiene rubbers may be made, for example, by organo lithium catalyzation in the presence of an organic hydrocarbon solvent. The polymerizations employed in making the rubbery polymers are typically initiated by adding an organolithium initiator to an organic polymerization medium that contains the monomers. Such polymerizations are typically carried out utilizing continuous polymerization techniques. In such continuous polymerizations, monomers and initiator are continuously added to the organic polymerization medium with the rubbery polymer synthesized being continuously withdrawn. Such continuous polymerizations are typically conducted in a multiple reactor system. Suitable polymerization methods are known in the art, for example as disclosed in US-B-4,843,120; 5,137,998; 5,047,483; 5,272,220; 5,239,009; 5,061,765; 5,405,927; 5,654,384; 5,620,939; 5,627,237; 5,677,402; 6,103,842; and 6,559,240.
Suitable solution polymerized styrene-butadiene rubbers are available commercially, such as Dow SLR 6410, Nippon Zeon NS 420. Such solution polymerized styrene-butadiene rubber may be tin- or silicon-coupled, as is known in the art. In one embodiment, suitable SSBR may be at least partially silicon-coupled.
Another component of the rubber composition is from 10 to 90 phr of polybutadiene. Suitable polybutadiene rubbers (BR) may be prepared, for example, by organic solution polymerization of 1,3-butadiene. The BR may be characterized, for example, by having at least a 90 percent cis 1,4-content and a glass transition temperature Tg in a range of from -95 to -105°C. Suitable polybutadiene rubbers are available commercially, such as Budene® 1207 from Goodyear.
A reference to glass transition temperature, or Tg, of an elastomer or elastomer composition, where referred to herein, represents the glass transition temperature(s) of the respective elastomer or elastomer composition in its uncured state or possibly a cured state in a case of an elastomer composition. A Tg can be suitably determined as a peak midpoint by a differential scanning calorimeter (DSC) at a temperature rate of increase of 10°C per minute.
In one aspect, the use of suitable SSBR and suitable polybutadiene of the specified composition may result in a rubber composition having at least two elastomer phases.
More specifically, between 5 and 40 weight percent of the elastomers may have a Tg lower than -95°C, namely the cis 1,4-polybutadiene, and between 60 and 95 percent by weight of the elastomers may have a Tg of higher than -40°C, namely the styrene-butadiene copolymer.
In this manner, it is considered herein that the relatively low Tg cis 1,4-polybutadiene elastomer is relatively incompatible with the high Tg styrene-butadiene copolymer elastomer as evidenced by their individual Tan delta peaks on a graphical presentation, or plot, of Tan delta versus temperature cured of the rubber composition within a temperature range of -120°C to 10°C.
Accordingly, the elastomers of the rubber composition may be present in at least two phases, comprised of a cis 1,4 polybutadiene phase and an additional phase comprised of said styrene/butadiene and/or tin- or silicon-coupled styrene-butadiene phase.
In particular, a graphical plot of Tan delta versus temperature curve within a broad range of -100°C to 10°C for the rubber composition of this invention may yield two peaks in the curve with one peak having its apex within a relatively low temperature range of -110°C to -70°C and a second peak with its apex within a higher temperature range of -35°C to +10°C.
Thus, one indication of the elastomer incompatibilities is the presence of the dual Tan delta peaks for the sulfur cured elastomer composition. The Tan delta values, with the included peaks in their curves, can be determined by dynamic mechanical testing of the cured compound by procedures well known to those skilled in such art.
Thus, the cured compounded rubber composition may exhibit at least two Tan delta peaks within the aforesaid temperature range. For the cured compounded rubber, it is considered herein that a combination of the Tan delta peak, at the lower temperature (e.g. -100°C to -50°C) for the low Tg elastomer (e.g. cis 1,4-polybutadiene), may suggest a promotion of improved resistance to abrasion property (i.e. improved treadwear for a tire) together with the second Tan delta peak, at the higher temperature (e.g. -30°C to 10°C) represented by the high Tg elastomer (e.g. the SBR), may suggest a promotion of higher hysteresis at temperatures within a range of -30°C to 0°C (i.e. higher tire tread traction), all of which is predictive of a better balance of such abrasion resistance and traction properties, particularly for a tire tread, than a cured rubber composition exhibiting only a single Tan delta peak within the aforesaid temperature range of -90°C to 10°C.
The rubber composition may optionally include from 0 to 20 phr of 3,4-polyisoprene rubber. The 3,4-polyisoprene rubber (3,4-Pl) is considered beneficial for the purpose of enhancing the tire's traction when it is used in a tire tread composition. The 3,4-PI is more fully described in U.S. Patent No. 5,087,668.
The rubber composition may also include from 30 to 70 phr of processing oil. Processing oil may be included in the rubber composition as extending oil typically used to extend elastomers. Processing oil may also be included in the rubber composition by addition of the oil directly during rubber compounding. The processing oil used may include both extending oil present in the elastomers, and process oil added during compounding. Suitable process oils include various oils as are known in the art, including aromatic, paraffinic, napthenic, and low PCA oils, such as MES, TDAE, and heavy naphthenic oils.
In one embodiment, the rubber composition includes a low PCA oil. Suitable low PCA oils include mild extraction solvates (MES), treated distillate aromatic extracts (TDAE), and heavy napthenic oils as are known in the art; see for example US-B- 5,504,135; 6,103,808; 6,399,697; 6,410,816; 6,248,929; 6,146,520; US-A 2001 /00023307; 2002/0000280; 2002/0045697; 2001/0007049; EP0839891; JP2002097369; ES2122917. Generally, suitable low PCA oils include those having a glass transition temperature Tg in a range of from -40°C to -80°C. MES oils generally have a Tg in a range of from -57°C to -63°C. TDAE oils generally have a Tg in a range of from -44°C to -50°C. Heavy napthenic oils generally have a Tg in a range of from -42°C to -48°C.
Suitable low PCA oils include those having a polycyclic aromatic content of less than 3 percent by weight as determined by the IP346 method. Procedures for the IP346 method may be found in Standard Methods for Analysis & Testing of Petroleum and Related Products and British Standard 2000 Parts, 2003, 62nd edition, published by the Institute of Petroleum, United Kingdom.

In one embodiment, the low PCA oils may be an MES, TDAE or heavy napthenic types having characteristics as identified in the following table.

	MES	TDAE	Heavy Naphthenic
Aromatics % (2140)	11 - 17	25 - 30	11 - 17
Spec. Gravity @ 15°C [kg/l]	0.895 - 0.925	0.930 - 0.960	0.920 - 0.950
Visc. 40°C (Cst)	150 - 230	370 - 430	350 - 820
Visc. 100°C (Cst)	13 - 17	16 - 22	17 - 33
Visc. Gravity Const.	0.825 - 0.865	0.860 - 0.890	0.840 - 0.870
Refractive Index	1.495 - 1.510	1.520 - 1.540	1.500 - 1.520
Tg [°C] / inflection	-60±3	-47±3	-45±3
Aniline Point [°C]	85 - 100
Pour Point [°C]	0 max	30 max	0 max
DMSO [%, IP 346]	<2.9	<2.9	<2.9
Flashpoint [°C]	> 220	> 240	> 240

In one embodiment, the low PCA oils may be an MES type that is a complex combination of hydrocarbons predominantly comprised of saturated hydrocarbons in the range of C₂₀ to C₅₀ obtained by (1) solvent extraction of heavy petroleum distillate; or (2) treating of heavy petroleum distillate with hydrogen in the presence of a catalyst; followed by solvent dewaxing. In one embodiment, the low PCA oil contains not more than 1 mg/kg of benzo(a)pyrene, and not more than 10 mg/kg total of the following polycyclic aromatic hydrocarbons: benzo(a)pyrene, benzo(e)pyrene, benzo(a)anthracene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene, dibenzo(a,h)anthracene, and chrysene.
Suitable MES oils are available commercially as Catenex SNR from Shell, Prorex 15 and Flexon 683 from ExxonMobil, VivaTec 200 from BP, Plaxolene MS from TotalFinaElf, Tudalen 4160/4225 from Dahleke, MES-H from Repsol, MES from Z8, and Olio MES S201 from Agip. Suitable TDAE oils are available as Tyrex 20 from ExxonMobil, VivaTec 500, VivaTec 180 and Enerthene 1849 from BP, and Extensoil 1996 from Repsol. Suitable heavy naphthenic oils are available as Shellflex 794, Ergon Black Oil C1, Ergon Black Oil C2, Ergon H2000, Cross C2000, Cross C2400, and San Joaquin 2000L. The oils may be available as the oil alone or along with an elastomer in the form of an extended elastomer.
The phrase "rubber or elastomer containing olefinic unsaturation" is intended to include both natural rubber and its various raw and reclaim forms as well as various synthetic rubbers. In the description of this invention, the terms "rubber" and "elastomer" may be used interchangeably, unless otherwise prescribed. The terms "rubber composition", "compounded rubber" and "rubber compound" are used interchangeably to refer to rubber which has been blended or mixed with various ingredients and materials, and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
The vulcanizable rubber composition may include from 50 to 150 phr of silica.
The commonly employed siliceous pigments which may be used in the rubber compound include conventional pyrogenic and precipitated siliceous pigments (silica). In one embodiment, precipitated silica is used. The conventional siliceous pigments employed in this invention are precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate.
Such conventional silicas might be characterized, for example, by having a BET surface area, as measured using nitrogen gas. In one embodiment, the BET surface area may be in the range of 40 to 600 square meters per gram. In another embodiment, the BET surface area may be in a range of 50 to 300 square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Volume 60, Page 304 (1930).
The conventional silica may also be characterized by having a dibutylphthalate (DBP) absorption value in a range of 100 to 400, alternatively 150 to 300.
The conventional silica might be expected to have an average ultimate particle size, for example, in the range of 0.01 to 0.05 micron as determined by the electron microscope, although the silica particles may be even smaller, or possibly larger, in size.
Various commercially available silicas may be used, such as, only for example herein, and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations 210, 243, etc; silicas available from Rhodia, with, for example, designations of Z1165MP and Z165GR and silicas available from Degussa AG with, for example, designations VN2 and VN3, etc.
The vulcanizable rubber composition may include from 1 to 30 phr of carbon black, crosslinked particulate polymer gel, ultra high molecular weight polyethylene (UHMWPE) or plasticized starch.
Commonly employed carbon blacks can be used as a conventional filler. Representative examples of such carbon blacks include N110, N121, N134, N220, N231, N234, N242, N293, N299, S315, N326, N330, M332, N339, N343, N347, N351, N358, N375, N539, N550, N582, N630, N642, N650, N683, N754, N762, N765, N774, N787, N907, N908, N990 and N991. These carbon blacks have iodine absorptions ranging from 9 to 145 g/kg and DBP number ranging from 34 to 150 cm³/100 g.
Other fillers may be used in the rubber composition including, but not limited to, particulate fillers including ultra high molecular weight polyethylene (UHMWPE), particulate polymer gels including but not limited to those disclosed in US-B 6,242,534; 6,207,757; 6,133,364; 6,372,857; 5,395,891; or 6,127,488, and plasticized starch composite filler including but not limited to that disclosed in US-B 5,672,639.
In one embodiment the rubber composition for use in the tire tread may contain a conventional sulfur containing organosilicon compound. Examples of suitable sulfur containing organosilicon compounds are of the formula:

Z - Alk - S_n - Alk - Z I

in which Z is selected from the group consisting of
where R⁶ is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; R⁷ is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbon atoms; Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is an integer of 2 to 8.
Specific examples of sulfur containing organosilicon compounds which may be used in accordance with the present invention include: 3,3'-bis (triethoxysilylpropyl) disulfide, 3,3'-bis (triethoxysilylpropy)I tetrasulfide, 2,2'-bis(methyl dimethoxysilylethyl) trisulfide, 2,2'-bis(methyl ethoxypropoxysilylethyl) tetrasulfide, 6,6'-bis(triethoxysilylhexyl) tetrasulfide, and 5,5'-bis(dimethoxymethylsilylpentyl)
In one embodiment, the sulfur containing organosilicon compounds are the 3,3'-bis(trimethoxy or triethoxy silylpropyl) sulfides. In one embodiment, the sulfur containing organosilicon compounds are 3,3'-bis(triethoxysilylpropyl) disulfide and 3,3'-bis(triethoxysilylpropyl) tetrasulfide. Therefore, as to formula I, Z may be
where R⁷ is an alkoxy of 2 to 4 carbon atoms, alternatively 2 carbon atoms; alk is a divalent hydrocarbon of 2 to 4 carbon atoms, alternatively with 3 carbon atoms; and n is an integer of from 2 to 5, alternatively 2 or 4.
In another embodiment, suitable sulfur containing organosilicon compounds include compounds disclosed in US-B- 6,608,125.
These sulfur containing organosilicon compounds are of the formula G - C(==O) - S - CH₂CH₂CH₂SiX₃ wherein each X is an independently selected RO- group wherein each R is independently selected from the group consisting of hydrogen, alkyl that may or may not contain unsaturation, alkenyl groups, aryl groups, and aralkyl groups, such moieties other than hydrogen having from 1 to 18 carbon atoms, and G is a monovalent alkyl of from 6 to 8 carbon atoms. In one embodiment, the sulfur containing organosilicon compounds includes 3-(octanoylthio)-1-propyltriethoxysilane, CH₃(CH₂)₆C(=O)-S - CH₂CH₂CH₂Si(OCH₂CH₃)₃, which is available commercially as NXT™ from GE Silicones.
In another embodiment, suitable sulfur containing organosilicon compounds include those disclosed in US-A- 2003/0130535.
These sulfur containing organosilicon compounds are of the formulas II or III

wherein: R is a methyl or ethyl group;
R' is identical or different and is a C₉C₃₀ branched or unbranched monovalent alkyl or alkenyl group, aryl group, aralkyl group, branched or unbranched C₂-C₃₀ alkyl ether group, branched or unbranched C₂-C₃₀ alkyl polyether group or R"'₃Si, where R"' is C₁-C₃₀ branched or unbranched alkyl or alkenyl group, aralkyl group or aryl group, R" is a branched or unbranched, saturated or unsaturated, aliphatic, aromatic or mixed aliphatic/aromatic divalent C₁-C₃₀ hydrocarbon group;
X is SH where n=1 and m=1, S where n=2 and m=1-10 and mixtures thereof, S(C==O) - R"' where n=1 and m=1 or H where n=1 and m=1;
In one embodiment, the sulfur containing organosilicon compound is of formula II, wherein R is ethyl, R' is C₁₂ - C₁₄ alkyl, R" is CH₂CH₂CH2, X is SH, n is 1 and m is 1. In one embodiment, the sulfur containing organosilicon compound is Si-363 from Degussa.
The amount of the sulfur containing organosilicon compound in a rubber composition will vary depending on the level of other additives that are used. Generally speaking, the amount of the compound will range from 0.5 to 20 phr. In one embodiment, the amount will range from 1 to 10 phr.
It is readily understood by those having skill in the art that the rubber composition would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, sulfur donors, curing aids, such as activators and retarders and processing additives, such as oils, resins including tackifying resins and plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants and peptizing agents. Representative examples of sulfur donors include elemental sulfur (free sulfur), an amine disulfide, polymeric polysulfide and sulfur olefin adducts. In one embodiment, the sulfur-vulcanizing agent is elemental sulfur. The sulfur-vulcanizing agent may be used in an amount ranging from 0.5 to 8 phr. Typical amounts of tackifier resins, if used, comprise 0.5 to 10 phr, usually 1 to 5 phr. Typical amounts of processing aids comprise 1 to 50 phr. Typical amounts of antioxidants comprise 1 to 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others, such as, for example, those disclosed in The Vanderbilt Rubber Handbook (1978), pages 344 through 346. Typical amounts of antiozonants comprise 1 to 5 phr. Typical amounts of fatty acids, if used, which can include stearic acid comprise 0.5 to 3 phr. Typical amounts of zinc oxide comprise 2 to 5 phr. Typical amounts of waxes comprise 1 to 5 phr. Typical amounts of peptizers comprise 0.1 to 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. The primary accelerator(s) may be used in total amounts ranging from 0.5 to 4. In another embodiment, combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts, such as from 0.05 to 3 phr, in order to activate and to improve the properties of the vulcanizate. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates.
The mixing of the rubber composition can be accomplished by methods known to those having skill in the rubber mixing art. For example, the ingredients are typically mixed in at least two stages, namely, at least one non-productive stage followed by a productive mix stage. The final curatives including sulfur-vulcanizing agents are typically mixed in the final stage which is conventionally called the "productive" mix stage in which the mixing typically occurs at a temperature, or ultimate temperature, lower than the mix temperature(s) than the preceding non-productive mix stage(s). The rubber composition may be subjected to a thermomechanical mixing step. The thermomechanical mixing step generally comprises a mechanical working in a mixer or extruder for a period of time suitable in order to produce a rubber temperature between 140°C and 190°C. The appropriate duration of the thermomechanical working varies as a function of the operating conditions, and the volume and nature of the components. For example, the thermomechanical working may be from 1 to 20 minutes.
The rubber composition may be incorporated in a variety of rubber components of the tire. For example, the rubber component may be a tread (including tread cap and tread base), sidewall, apex, chafer, sidewall insert, wirecoat or innerliner. In one embodiment, the compound is a tread.
The pneumatic tire of the present invention may be a race tire, passenger tire, aircraft tire, agricultural, earthmover, off-the-road, or truck tire. In one embodiment, the tire is a passenger or truck tire.
Vulcanization of the pneumatic tire of the present invention is generally carried out at conventional temperatures ranging from 100°C to 200°C. Any of the usual vulcanization processes may be used such as heating in a press or mold, heating with superheated steam or hot air. Such tires can be built, shaped, molded and cured by various known methods.
The following examples are presented for the purposes of illustrating the present invention. All parts are parts by weight unless specifically identified otherwise.

EXAMPLE I

In this example, eight rubber compounds are compared. Samples 1 and 2 represent control samples typical of the prior art. Sample 3 is representative of the present invention.

The elastomers were compounded in a three-stage mix procedure with standard amounts of conventional curatives and processing aids as indicated in Table 1, and cured with a standard cure cycle. Cured samples were evaluated for various physical properties following standard tests protocols as indicated in Table 2.

Table 1 Base Compound Recipe

First Non-Productive Mix Step
Elastomers¹	100
Oil²	50
Silica³	95
Coupling agent⁴	15.2
Waxes⁵	1.5
Stearic acid	2.5
Antidegradant⁶	0.75

Second Non-Productive Mix Step
Antidegradant⁶	1.75

Productive Mix Step
Antidegradant⁶	1
Zinc oxide	3.5
Sulfur	1.4
Accelerators⁷	5.5
Coupling agent⁸	2

¹From Table 2
²Including elastomer extender oils as defined in Table 2 and added process oil
³Z1165MP from Rhodia
⁴50 percent bis(triethoxypropylsilyl) disulfide on carbon black
⁵Microcrystalline and paraffinic
⁶P-phenylenediamine type
⁷Sulfenamide and guanidine type
⁸50 percent bis(triethoxypropylsilyl) tetrasulfide on carbon black

Table 2 Elastomers and Physical Properties of Vulcanizates

Sample	1	2	3	4	Control	6 Control	7 Control	8 Control
Polybutadiene⁹	40	30	20	10	0	0	20	40
SSBR, high styrene¹⁰	60	70	80	90	100	0	0	0
SSBR, med styrene¹¹	0	0	0	0	0	100	80	60

Physical Properties
Shore A	66.9	66.3	66.2	66.2	66.8	60.7	63.5	62.5
Rebound 0°C	12.4	10	7.6	6	5	4.4	6	12.4
Rebound 23°C	31.6	28.4	25.8	23.6	19.4	13.4	23	23
Rebound 100°C	62.4	61.6	61.4	62.8	61.8	60.4	61.2	62.8
Elongation, %	436	469	417	458	440	389	385	392
True Tensile, MPa	93.3	109	89.3	104.4	104.1	81.7	76.5	76.4
Modulus@100%	2.38	2.36	2.41	2.39	2.6	2.22	2.16	2.1
Modulus@200%	6.35	6.46	6.69	6.65	7.35	6.87	6.31	5.96
Modulus@300%	12.1	12.2	12.7	12.4	13.6	13.3	12.5	12
Tensile Strength, MPa	29.5	35.4	27.7	34.1	33.9	24.1	22.2	22
Drum Abrasion (mm³⁾	86	93	113	129	149	182	135	99

⁹Budene® 1207, from The Goodyear Tire & Rubber Company
¹⁰SE SLR 6410, partially silicon-coupled, solution-polymerized styrene butadiene rubber extended with 50 phr aromatic oil, typical properties reported as 40 percent by weight of styrene and 25 percent by weight vinyl (of butadiene segments), Tg = -32.9°C, Mooney ML 1+4 (100 °C) = 65.0, from the Dow Chemical Company
¹¹SE SLR 4610, partially silicon-coupled, solution-polymerized styrene butadiene rubber extended with 37.5 phr aromatic oil, typical properties reported as 25 percent by weight of styrene and 63 percent by weight vinyl (of butadiene segments), Tg = -25.2°C, Mooney ML 1+4 (100°C) = 45.0, from the Dow Chemical Company.

As can be seen from Table 3, samples 1 through 4 representative of the present invention, show an improved balance between wet grip and wear as compared with the control samples 5 through 8. The Figure illustrates the improved balance of wet grip vs. wear balance. As seen in the Figure, samples 1 through 5 with the immiscible blend of polybutadiene and SSBR show a lower wear for a given wet grip as compared with the miscible blend of polybutadiene and SSBR (samples 6 through 8).

Claims

A pneumatic tire having a tread comprising a vulcanizable rubber composition comprising, expressed as parts by weight per 100 parts by weight of elastomer (phr),
(A) 60 to 95 phr of solution-polymerized styrene-butadiene rubber with a bound styrene content of from 35 to 45 percent by weight, a vinyl 1,2 content of from 15 to 30 percent by weight based on the butadiene content, and a Tg of from -40°C to -20°C;

(B) 5 to 40 phr of cis-1,4 polybutadiene having a Tg of from -95°C to - 105°C;

(C) 50 to 150 phr of silica;

(D) 30 to 70 phr of processing oil; and

(E) 0.5 to 20 phr of a sulfur containing organosilicon compound.
The pneumatic tire of claim 1, where said styrene-butadiene rubber has a bound styrene content of from 38 to 42 percent by weight.
The pneumatic tire of claim 1 or 2, wherein said styrene butadiene rubber has a vinyl 1,2 content of 20 to 25 percent by weight based on the butadiene content.
The pneumatic tire of at least one of the previous claims, wherein said vulcanizable rubber composition further comprises from 1 to 30 phr of a filler selected from carbon black, starch/plasticizer composite, and crosslinked particulate polymer gel.
The pneumatic tire of at least one of the previous claims, wherein said processing oil comprises a low PCA process oil having a polycyclic aromatic content of less than 3 percent by weight as determined by the IP346 method, said low PCA oil being selected from mild extraction solvates (MES), treated distillate aromatic extracts (TDAE), or heavy napthenic oils.
The pneumatic tire of at least one of the previous claims, wherein said solution polymerized styrene-butadiene rubber is at least partially silicon coupled.
The pneumatic tire of at least one of the previous claims, wherein said sulfur containing organosilicon compound comprises at least one of 3,3'-bis(triethoxysilylpropyl) disulfide or 3,3¢-bis(triethoxysilylpropyl) tetrasulfide, or 3-(octanoylthio)-1-propyltriethoxysilane.
The pneumatic tire of at least one of the previous claims, wherein said sulfur containing organosilicon compound comprises a compound of the formula:

Z - Alk - S_n - Alk - Z I

in which Z is selected from the group consisting of
where R⁶ is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; R⁷ is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbon atoms; Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is an integer of 2 to 8.
The pneumatic tire of at least one of the previous claims, wherein said sulfur containing organosilicon compound comprises a compound of the formula
G-C(=O)-S-CH₂CH₂CH₂SiX₃, wherein each X is an independently selected RO group, wherein each R is independently selected from the group consisting of hydrogen, alkyl that may or may not contain unsaturation, alkenyl groups, aryl groups, and aralkyl groups, such moieties other than hydrogen having from 1 to 18 carbon atoms, and G is a monovalent alkyl of from 6 to 8 carbon atoms.
The pneumatic tire of at least one of the previous claims, wherein said sulfur containing organosilicon compound comprises at least one compound of formula II or III

wherein: R is a methyl or ethyl group;
R' is identical or different and is a C₉-C₃₀ branched or unbranched monovalent alkyl or alkenyl group, aryl group, aralkyl group, branched or unbranched C₂-C₃₀ alkyl ether group, branched or unbranched C₂-C₃₀ alkyl polyether group or R"'₃Si, where R"' is C₁-C₃₀ branched or unbranched alkyl or alkenyl group, aralkyl group or aryl group, R" is a branched or unbranched, saturated or unsaturated, aliphatic, aromatic or mixed aliphatic/aromatic divalent C₁-C₃₀ hydrocarbon group;
X is SH where n=1 and m=1, S where n=2 and m=1-10 and mixtures thereof, S(C==O) - R"' where n=1 and m=1 or H where n=1 and m=1;
R" may mean CH₂, CH₂CH₂, CH₂CH₂CH₂, CH₂CH₂CH₂CH₂, CH(CH₃), CH₂CH(CH₃), C(CH₃)₂, CH(C₂H₅), CH₂CH₂CH(CH₃), CH₂CH(CH₃)CH₂ or